This invention relates to an oxygen permeable membrane which permits oxygen gas to pass through while substantially blocking water vapor. The present invention has application in hydrogen-oxygen electric fuel cells, metal-air battery cells and oxygen sensoring devices.
In the prior art, gas diffusion electrodes have been used for air electrodes in various fuel cells, air-metal cells, such as air-zinc cells, and Galvanic oxygen sensors. Thick porous electrodes having distributed pores with a uniform diameter were used as the gas diffusion electrode. In recent years, however, electrodes having a two-layer structure have been used. The electrode comprised a porous electrode body with an electrochemical reduction function for oxygen gas (a function for ionizing oxygen) and a current collector function. The electrode also had a thin water repellent layer deposited integrally on the gas-side surface of the electrode body.
In such electrodes, the electrode body may be formed by a conductive powder, a porous metallic body, a porous carbon body or a non-woven carbon fabric material. This may be accomplished by use of a binder such as polytetrafluoroethylene. Such conductive powders may be selected from among active carbon powders carrying a nickel tungstate with a low reduction overvoltage to oxygen gas, a tungsten carbide coated with palladium, cobalt, nickel, silver, platinum or palladium.
The aforementioned water repellent layer, which is deposited integrally on the gas-side surface of the electrode body, is a porous thin membrane which comprises a fluorine containing a resin such as polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer or ethylene-tetrafluoroethylene copolymer. The membrane may also be comprised of a resin, such as polypropylene, in the form of a porous material including, for example, a sintered powder material having a particle size of from 0.2 to 40 .mu.m; a paper-like non-woven fabric material prepared by heat treatment of fibers comprising polypropylene resin or by a similar woven fabric material; a powder material wherein the polypropylene resin is partially replaced by a fluorinated graphite; a film material prepared by rolling fine powder together with a pore-increasing agent or a lubricant oil followed by heat treatment or a film material prepared by rolling without being followed by heat treatment. Such materials are disclosed in Japanese Patent Publication No. 44978/1973.
In an air electrode having the structure as described above, however, the water repellent layer deposited on the gas-side surface of the electrode body is impervious to electrolyte but is not impervious to air and water vapor in the air. For this reason, water vapor in the air may penetrate the electrode body through the water repellent layer and dilute the electrolyte or the water in the electrode may otherwise dissipate through the water repellent layer, thereby increasing electrolyte concentration. As a result, the concentration of the electrolyte will fluctuate and it will thus be impossible to maintain a stable discharge characteristics over a long period of time.
When carbon dioxide gas in the air penetrates the electrode body through the water repellent layer and is absorbed by an active site (a porous portion of the electrode body), the electrochemical reducing function of the active site to oxygen gas is reduced at the point of absorption. Thus, the heavy-load discharge capability of the cell is adversely effected. Moreover, when an alkaline electrolyte is used, there will be a change in the properties of the electrolyte, a reduction in the concentration of the electrolyte and, if the cathode is zinc, passivation of the zinc cathode. Furthermore, heavy-load discharge may be hindered because the area of electrochemical reduction is reduced by the production of carbonate in the active site which blocks the pores. These factors lead to a decline in the performance of the cell from its design rating after the cell is stored or used for a long period of time.
In order to overcome the aforementioned disadvantages, there has been proposed a cell in which a water repellent layer for the air electrode is provided on the gas side (air side) thereof with a layer comprising a water-absorbing agent, such as calcium chloride, or a carbon dioxide gas-absorbing agent, such as a hydroxide of an alkaline earth metal. Such a cell is disclosed in Japanese Patent Publication No. 8411/1973. This type of cell can prevent the above-mentioned problems to some extent. However, when the absorbing agent has been saturated with water or carbon dioxide gas, it becomes wholly ineffective.
There have also been attempts to laminate an oxygen permeable membrane on the abovementioned water repellent layer. Such a membrane is disclosed in Japanese Patent Publication No. 26896/1973. However, a sufficiently effective oxygen gas permeable membrane has not been developed thus far.